fix: rename the strict exit code, cover the config path, refresh the docs

Copilot review round 3.

- Rename the `--strict` failure from "invalid" to "unportable"
  (`EXIT_CODES.SCHEMA_UNPORTABLE`, envelope `schema_unportable`, and the
  message text). The module argues at length that these schemas ARE
  valid JSON Schema and are merely refused by some clients, so a
  machine-readable code of `schema_invalid` told automated callers the
  opposite. Done now, while nothing depends on it.

- Add a fixture-path integration case. Every other test built a
  `ServerConfig` by hand, so `resolveConfig`'s `rawToolSchemas`
  forwarding was uncovered and `unportable-schemas-http.json` was loaded
  by nothing — the shipped fixture could have lost its overrides with
  the suite still green. It now goes through loadConfig/resolveConfig
  and asserts the exact per-tool verdict the README claims.

- Refresh the CLI README's sample output, which still showed the
  pre-round-2 wording and fixture, and so dropped the contract warning
  that rewording existed to add. Captured from a real run rather than
  written by hand, plus a paragraph making the suggestion pattern
  explicit.

- Drop the self-referential "[Try it](#unportable-tool-schemas)" link in
  the root README, which pointed back at its own section.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UL9retfvAXRgvi6EWk4SY4
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/cli/__tests__/schema-lint-report.test.ts

@@ -128,8 +128,11 @@ describe("emitResult — schema lint wiring (#1005)", () => {
     });
     await expect(promise).rejects.toBeInstanceOf(CliExitCodeError);
     await promise.catch((e: CliExitCodeError) => {
-      expect(e.exitCode).toBe(EXIT_CODES.SCHEMA_INVALID);
-      expect(e.envelope?.code).toBe("schema_invalid");
+      expect(e.exitCode).toBe(EXIT_CODES.SCHEMA_UNPORTABLE);
+      // Not `schema_invalid`: the lint reports schemas that ARE valid JSON
+      // Schema and are merely unportable, so the code an automated caller
+      // branches on must not claim otherwise.
+      expect(e.envelope?.code).toBe("schema_unportable");
     });
     // The result still goes to stdout; only the report is on stderr.
     expect(JSON.parse(stdout)).toEqual(DIRTY_LIST);